Automatic tuning device for radio receivers



Feb. 11, 1941. Hl. JACKSON Y 2,231,386

AUTOMATIC TUNING DEVICE FOR RADIO RECEIVERS BY MM Arron/vir H. JACKSON Feb. 11, 1941.

AUTOMATIC TUNING DEVICE FOR RADIO RECEIVRS Filed Feb. 25, 1955 2 SheetS-Sheet 2 Patented Feb. 11, 1941 UNITE STATES PATENT OFFICE AUTOMATIC TUNING DEVICE FOR RADIO RECEIVERS Application February 23, 1935, Serial No. '7,778 In Great Britain February 24, 1934 6 Claims.

The present invention relates to automatic tuning devices for radio receivers. The invention is concerned with tuning control apparatus which operates indirectly to adjust the setting of a tuning member of the receiver. An adjustment may be regarded as indirect for the purposes of this specication when it is eiected otherwise than by the agency of a direct rigid, frictional or like mechanical coupling between the adjusting means and the tuning member.

On account of the large numbers of broadcasting stations having wavelengths within the restricted compass of the broadcast wavebands, the selectivity of broadcast receivers of good sensitivity must be made high, so that a desired ignal may be received without adjacent channel interference. It is found, however, that the operation of tuning a highly selective receiver is one requiring considerable accuracy, since mistuning may result in serious distortion; when such a receiver is not accurately tuned, adjacent channel interference and sideband cutting, among other influences, may mar the reception of the desired signal.

The need for highly critical tuning is an important factor in the design of indirect tuning control apparatus; in a case, for example, in which tuning is effected by the adjustment of a Variable condenser, it may be necessary to adjust the condenser to within a fraction of a degree of arc of the correct setting in order to avoid distortion, and indirect control apparatus capable of such an accuracy is complicated, diicult to set up, and expensive to manufacture.

It is the principal object of the present invention to provide wireless or like apparatus, comprising a receiver and indirect tuning control means therefor, in which a desired accuracy of tuning may be simply achieved, although the indirect control means,I of itself, may be incapable of effecting such accurate tuning.

According to the present invention, wireless or like apparatus comprises a signal receiver having an adjustable tuning member, and tuning control means adapted to operate indirectly for effecting adjustment of said member, said apparatus comprising, at the receiver, line-tuning means adapted to operate automatically to compensate for lack of precision in the operation oi said tuning control means.

The tuning control means referred to in the preceding paragraph may be arranged at the receiver itself, or they may comprise a control member arranged at a remote point; they are preferably adapted to tune the receiver to any of a number of pre-selected tuning positions. I'he fine-tuning means operate to provide what will be referred to as pull-in tuning, that is to say, when the receiver has been approximately tuned 5 to a desired signal, the fine-tuning means operate to pull the receiver exactly into tune with that signal.

Various embodiments of the present invention will now be described by way of example with 10 reference to the accompanying drawings, in which Fig. 1 is a diagrammatic drawing of a form of push-bottom tuning control device which may form a part of apparatus according to this invention,

Fig. 2 is a circuit diagram of a wireless receiver incorporating means for providing pull-in tuning, and suitable for use in association with the tuning control device of Fig. 1,

Fig. 3 is a circuit diagram of a preferred form of apparatus according to the invention, incorporating a superheterodyne receiver having pull-in tuning, and means for controlling the tuning of the receiver from a remote point, and 20 Fig. 4 illustrates a further form of apparatus according to the invention.

Referring to Fig. l, the tuning condenser I of a wireless receiver comprises three condenser sections 2, 3 and l having their rotor elements 30 mounted upon a common control spindle 5. The spindle 5 carries a pinion 5A which co-operates with a pinion 1A, the latter being coupled to a knob l' which is mounted on the cabinet 6 of the receiver and provides manual tuning.

Keyed to the spindle 5 is a pinion 8 which co-operates with a rack 9; the latter bears a rod I B which projects through the side of the cabinet (i and terminates in a push-key II. A helical spring I2 is connected between a boss I4 on the cabinet Wall and a boss I3 secured to the rod IB, the action of this spring being to tend to push the rack 9 to the left, and to turn the rotor varies of condenser I into the maximum capacity position. When the key II is urged to the right to its full extent, against the action of spring I2, a catch I4A, which will be referred to as the reset catch, engages the boss I3 and holds key II in this position; in these circumstances, the condenser I is at its minimum capacity setting.

The spindle 5 also bears a radial arm I5 which is arranged to co-operate with a plurality of station selector keys, of which only two, numbered I6 and I'I, are shown for simplicity. The station 55 Lil.

' inal accuracy is lost.

v semicircle having the spindle 5 as centre.

keys bear stops numbered I8 and I9 respectively, which project through the cabinet wall and are normally held out of the path of the arm I5 by suitable springs 29 and 2| respectively.

The station keys and their associated stops are so constructed and arranged that when any one of the keys is depressed, and the arm I5 is rotated until it abuts against the stop associated with that key, the setting of the condenser I corresponds to the tuning position of the station represented by the key in question. A separate selector key is provided for each of any number of stations which it may be desired to pre-select.

The operation of the arrangement is as follows. Assume rstly that the condenser I is held in the minimum capacity setting by the re-set catch IA, and that the station key I6 is depressed to bring its associated stop I8 into the path of the arm I5, as shown: as the key I6 is depressed a shoulder ISB formed on stop I8 comes into contact with plate IBCD andurges it forward, the plate IliBCD being secured to a sliding boss 5C on spindle 5. Plate IGBCD is coupled to the re-set catch IiiA, and a spring IQE is provided to hold catch lli-A normally in engagement with boss I3. When key I6 is fully depressed, catch MA is released; the spindle 5 commences to turn under the thrust of the spring I2, and the arm I5 is carried around with the spindle until it is brought to rest by stop I3 of key I6. The enlarged end ISA of stop I8 prevents the latter from disengaging from the arm I5. If the positioning of the stop I8 has been sulciently accurately predetermined, the receiver is now tuned to the station represented by key I6.

If now it is desired to receive the stationrepresented by key Il, for example, the re-set key II is rst depressed to its full extent; this operation causes the stop IS of key I6 to be released, and the re-set catch re-engages boss I3. Station key II is now depressed, and a similar cycle of operations takes place.

The arrangement described is difiicult to adjust for accurate tuning, and if the station key stops or the arm I5 wear or become deformed, the orig- As is more fully explained hereinafter, pull-in tuning is accordingly provided to compensate for lack of precision on the part of the tuning control.

' The tuning control device described is an example of a simple indirect tuning device, and manymodications and elaborations of the arrangement described will occur to those versed in the art; for example, the station keys may be ari ranged on the receiver cabinet' in any desired way, provided that the station-key stops are so shaped and disposed as to arrest the spindle arm I5 in the desired positions. veniently be disposed on the circumference of a The re-set key may be dispensed with, and the control knob I employed for restoring the condenser I to the minimum capacity setting; it may be arranged that when catch IGA is latched behind boss I3, the knob 'I may be freely rotated, to allow the receiver to be readily tuned by hand.

A suitable indicating lamp 5B is coupled to spindle 5 for rotation therewith, and each station key is disposed near to a hole in the cabinet (one such hole, iA, being shown), the arrangement being such that when the receiver is tuned te the st`t1'on represnvd by key i'i, for example, the lmnp lB is di.p.ised boh';.d t e hole, NA, associated withvtliat key. I

The keys may con- The plate IiBCD is in three parts; a plate IB is rigidly secured to the sliding boss 5C, and carries a plate Ill-C of insulating material, on the face of which is arranged a metal plate IQD, the whole structure forming part of a switch for wave-range changing purposes. The arrangement is as follows: Assuming that station key I'I corresponds to a long wave station, while key Iii corresponds to a station on a shorter wave band, when key I'I is depressed, it is arranged that the shoulder ISB engages metal plate ID; when key I6 is depressed, however, it is arranged that shoulder ISB passes through an opening in plate IiD and engages insulation plate llC. This contact may be arranged to short-circuit an inductance coil for wave-range changing, either directly or through the agency of a relay.

Now it is known that, in order to obtain reception free from interference and distortion, it may be necessary to tune the receiver to a desired station with an error of not more than a fraction of a degree of arc. The tuning device described is only capable of this degree of accuracy if very great care is exercised in setting it up, and even so, is not capable of taking account of small changes in the wavelengths of the stations to which selector keys are allotted. Furthermore, if in the course of time the spindle arm or the stationkey stops become deformed or worn, the original accuracy of tuning is lost.

Accordingly, the receiver is provided with means for eiecting pul1-in tuning to compensate for deciencies on the part of the tuning control device. Fig. 2 shows a wireless receiver of the so-called straight type, incorporating pull-in tuning' means. It will be assumed that the receiver shown in Fig. 2 incorporates the tuning control device described with reference to Fig. 1.

Referring to Fig. 2, a wireless receiver comprises an input tunable circuit 22 having a variable tuning condenser 23 shunted by a Vernier, or fine-tuning condenser 24. oscillations from the circuit 22 are fed to a high-frequency pentode amplifying valve 25 having its anode connected through a high-frequency choke coil 26 to the positive terminal 2l' of a source of anode current illustrated generically by battery 21A; the negative terminal of the source, which may comprise a suitable rectier instead of the battery illustrated,

is connected to the grounded side of the receiver circuit.

The valve 25 feeds the primary winding 29 of a high-frequency transformer 30 through a coupling condenser 3l. The secondary winding of transformer 3v is in two halves comprising coils 32 and 33; the mid-point of the secondary winding is earthed.

Coil 32 is shunted by a tuning condenser 3Q Which, in turn, is shuntedby the Vernier condenser 35, and coil 33 is shunted by tuning condenser S and Vernier condenser 37. oscillations set up across the two halves oi the secondary winding of transformer 3G are fed through coupling condensers 38 and 39 to the control grids of two valves 49 and lil, which have their cathodes connected together and to earth.

Received signal oscillations undergo full wave diode rectication in the grid circuits of the valves l0 and 4I. The grid circuit of valve "IIJ includes a leak resistance e2 in shunt with a highfrequency choke 43, and the grid circuit of valve '"5 comprises leak resistance M and choke 45. The modulation frequency oscillations set up across resistances 42 and M, are fed through condensers 45 and 4l to a Volume control potential divider resistance 48 connected asr shown.

A variable tapping point in resistance i8 is connected through a coupling condenser 1&9 tothe control grid of an output pentode Valve 55; the latter valve serves to amplify the modulation frequency oscillations and has in its anode circuit the primary winding of an output transformer 5i adapted to feed a loudspeaker, not shown.

The condensers 23, 35 and 33 are ganged to a common spindle for simultaneous actuation, and may be considered as corresponding to condensers 2, 3 and 4 of Fig. l; Vernier condensers 2e, 35 and 31 are also ganged to a common shaft. Means of any suitable kind such as the arrangement al-u ready described in connection with Fig. 1, are provided for achieving wave-band switching, and decoupling condensers and resistances are provided in known manner throughout the circuit. More than one high-frequency amplifying stage may be provided if desired.

Connected between the anodes of the valves to and il is a coil 52, which is mounted in a magnetic field in such a manner as to be capable of rotating freely therein, and is arranged to control the settings of Vernier condensers 24, 35 and 3l as shown by the dotted line 52 in Fig. 2. The ends of this coil are connected through load resistances 53 and 5t to the ends of a potential divider resistance 55, a variable tapping point in which is connected through a high-frequency filter 56 to the terminal 21 of the anode current source. y

It is arranged that at any setting of the condensers 23, 3d, 36 the circuit 32, 35 is tuned to a frequency slightly above the frequency to which circuit 22 is tuned, while circuit 33, 33 is tuned to] the same extent below the frequency of circuit 22. If now the circuit 22 is accurately tuned to a signal, the signal potential dilerence set up across circuit 32, 315 is the same as that set up across circuit 33, 33, and, assuming the characteristics of valves 45 and il to be matched, the anode currents in these valves are the same. In these circumstances, that is to say, when circuit 22 is accurately tuned, it is arranged that no current flows in the coil 52; this is achieved by adjusting the tapping point in resistance 55 until the bridge constituted by elements 52, 53, 5t and 55 is balanced.

The coil 52 is mechanically coupled to the common shaft to which Vernier condensers 23, 35 and 3l are ganged, and it is arranged that when the anode current of valve lil exceeds that of valve tl, the capacities of the Vernier condensers are increased, while when the anode current of valve 40 is less than that of valve 4I, the Vernier condenser capacities are decreased.

Suppose now that a certain station key is incorrectly positioned so` that, when depressed, it stops the main tuning condenser 23 at a position such that its capacity is slightly lower than the correct setting; then the circuit 33, 35 is more accurately tuned to the desired station than is circuit 32, 35 and more anode current flows in valve ri than in valve 4|.

A potential difference is thus set up across coil 52, and current flows in this coil in such sense that the capacity of the Vernier condensers is increased. This has the effect of reducing the frequency to which circuit 22 is tuned, thus tending to correct for the initial tuning error; furthermore, the frequencies to which circuits 32, 3d and 33, 36 are tuned are decreased, and when the correct tuning condition is attained, these circuits are equally mistuned, the potential difference across coil 52 falls to Zero, and no further adjustment of the Vernier condensers takes place. Thus the receiver automatically takes up the correct tuning setting, that is, pull-in tuning is obtained; furthermore, correct tuning is maintained if the station frequency alters slightly.

Pull-in tuning is achieved in a similar manner when a station key which tunes the receiver to a frequency lower than the station frequency is operated. In this case, circuit 32, 33 is more accurately tuned than circuit 33, 3S, and current flows in coil 52 in such a sense as to tend to reduce the Vernier condenser capacities.

Many modifications of the pull-in tuning arrangements described are possible, and will readily occur to those versed in the art. For example, the modulation frequency output may be taken from the anode circuits of the valves 45 and 4| instead of from the grid circuits, and in order to ensure correct matching and equivalent deterioration, the valves 40 and 4I may be replaced by a double triode valve having a common cathode. If desired, a separate detector may be employed, the valves 4) and il being employed solely for operating the Vernier condensers. The output of valves 45 and di may be amplified in direct coupled ampliers so as to ensure that an adequate force is produced by coil 52 to rotate the Vernier condensers. The coil 52 may be replaced by a single centre-tapped coil, or by two coils oppositely wound. If the main tuning condenser spindle is sufficiently light and freely rotating, and if the tuning control means are of a suitable kind, the Vernier condensers may be omitted, and the condenser spindle may then be driven by the coil 52 itself; alternatively, a reversible electric motor may be provided to rotate the condenser spindle.

The present invention may also be applied to g receivers having tuning means operable from a remote point; since pull-in tuning is provided, the remote control means need not be arranged to provide a very high degree of accuracy, and many simple remote control devices may therefore be employed. Fig. 3 shows a preferred form of wireless apparatus according to the present invention, which incorporates remote tuning control means.

Referring to Fig. 3, a superheterodyne receiver comprises a high-frequency amplifying stage 57, comprising a Variable condenser 58, a heptode valve 59 which acts as local oscillator and mixer, an intermediate-frequency amplier 60, and a second detector and output stage 6i. The first control grid, that is, the inner grid of the heptode valve 59, is connected through a leak resistance 62 shunted by a grid condenser E3, and through a parallel resonant circuit 64, to the chassis, which is indicated by the line 65. An electrode 65, which serves as an anode for the generation of local oscillations, is connected through a coil Eil to a point 63 in a source of anode current indicated generically by battery 68'; the negative terminal of the source is returned to the chassis and is earthed, and other points in the figure which are marked with a positive sign are connected to points in the source at suitable positive potentials. The coil 6l is coupled to the coil 65 of resonant circuit 64 and the variable condenser 'i5 of that circuit is ganged to the variable condenser 55, in the signal frequency stage 5l of the receiver, so that only one tuning control is necessary.

The second control grid of the heptode is connected to the output terminal 1| of the signal frequency stage 51 of the receiver.

The anode circuit of the heptode 59 comprises a parallel resonant circuit 12 tuned to the intermediate frequency; a coupling coil 13 is coupled to the tuned circuit 12, and feeds the intermediate frequency amplifier 88. A coupling condenser 14 is connected between the anode of heptode 59 and the control grid of auxiliary intermediate frequency amplifying valve 15. The cathode of this valve is connected to the chassis through a biasing resistance 18 shunted by a by-pass condenser 11, and its anode is connected through an intermediate-frequency choke coil 18 to the anode current source.

The anode of the valve 15 is connected to the chassis through a, series resonant circuit comprising a coil 19 and a condenser 88; the latter is preferably made adjustable. A further coil 8| is coupled to the coil 19. The Values of the circuit elements 19, 88, 8| and 82 are made such that the potential difference set up at the exact intermediate frequency across coil 8| is equal to that set up across condenser 98 at that frequency.

The junction point of condenser and coil 19 is connected through a condenser 82 to the anode 83 of a double-diode valve 85 having a cathode 84, and the coil 8| is connected between the anode 88 and cathode 84 of valve 85. The diode '83, 84 has a load resistance 81 of value such as 5 rnegohms for example, and diode 88, 84 has a load resistance 88 of substantially the same high value. The load resistances 81 and 88 are shunted by by-pass condensers 89 and 98 respectively, which may have a capacity of 0.01 microfarad. An intermediate frequency choke coil 9| is connected in shunt with diode 83, 84 and resistance 81 to complete the external D. C. circuit of diode 88, 88.

The end A of load resistance 81 remote from the cathode 88 of valve 85 is connected to the outer control grid of a hexode valve 92, having its anode connected through a resistance 93 to the anode current source. The end A of the resistance 88 remote from the cathode 84 of valve 85 is connected to the chassis 65, and the cathode of hexode 92 is connected through a biasing resistance 94 shunted by a by-pass condenser 95 to the chassis.

The inner control grid of hexode 92 is connected to that side of condenser 18 which is not directly connected to the chassis, through a condenser 98. A grid leak 99 is connected as shown. A condenser 96 is connected between the anode of hexode 92 and the inner grid thereof; the function of this condenser will be explained hereinafter. Condenser 98 may have a capacity of the order of 18 micro-microfarads.

The common tuning shaft of condensers 58 and 10 is rigidly coupled to the armature of a reversible electric motor |88, which is arranged to tune the receiver under the control of an operator at a remote point. In this gure, parts to the left of the dotted line X-X are assumed to be at the remote point. The terminal Y is assurned to be connected to terminal Y', so that the output of the output stage of the receiver is fed to the remote loudspeaker LS. The latter is associated with a volume control VC, and a loudspeaker (not shown) is also rprovided at the receiver itself.

The operation of the motor |88 is controlled from the remote point by means of a Wheatstone l bridge comprising two potential divider resistances |8| and |82 which have their adjustable tapping points |83 and |84 connected together through the winding of a polarised relay |85 of known kind. Across the other diagonal of the bridge is connected a source of current such as battery |86.

The motor |88 has an armature winding |81 which is fed with alternating current from a suitable source such as the mains, and two separate stator windings |88 and |89, which are energized through phasing condenser H8. The direction in which the armature moves depends upon which of windings |88 and |89 is energised, these windings being connected and arranged in such a manner that by disconnecting one of them and switching in the other, the motor is reversed.

Potential divider resistance |8| at the remote point has a movable arm for varying the position of tapping point |83, this arm being associated with a tuning scale engraved, for example, with the names of broadcasting stations; the corresponding movable arm of potential divider resistance |82 at the receiver is mechanically coupled to the armature of motor |88.

When the bridge comprising resistances |8I, |82 is balanced, there is no current in the winding of relay |85, and the contact-makers Ill, ||2 are in the mid-positions as shown; in these circumstances, the motor |88 is deenergised. Suppose now the movable arm of resistance |8| is moved to a fresh position corresponding to a desired station. The balance of the bridge is disturbed, current flows in the winding of relay |85 and contact makers and ||2 move to the right or left. The armature of relay |85 is polarised, as already stated, and thus if current iiows in one direction in the relay winding, motor windings |81 and |88 are energised, whilst if current ows in the other direction, motor windings |81 and |89 are energised.

The armature of motor |88 thus commences to turn, and the tuningr of the receiver undergoes adjustment. Furthermore, the movable arm of resistance |82 is caused to move in such a sense as to restore the balanced condition of the bridge, and when this condition is attained, relay |85 is released, and the motor I8 is de-energised.

The remote control arrangement described can be made to provide approximately correct remote tuning, but cannot, for a number of reasons, give a high degree of accuracy; among these reasons are, rstly, that the sensitivity of the relay |85 is limited, so that the motor circuit is likely to be opened before the bridge reaches the balanced condition, and, secondly, that the motor |88 does not stop instantaneously when it is de-energised. Pull-in tuning is accordingly provided to compensate for the inadequacies of the remote control means. The way in which pull-in tuning is obtained will now be considered.

It has already been explained that the potential differences set up across coil 8| and condenser 88 at the intermediate frequency, that is, where a station is accurately tuned in, are equal. 'Ihe diodes 83, 84 and 86, 84 have very high load resistances; thus the potential differences set up across the load resistances are very nearly equal to the peak voltages applied to the diodes from coil 8| and condenser 88 respectively, the losses in the diodes being negligibly small. It is therefore not essential that the two diodes should be accurately matched, since the rectied voltage in each case is substantially independent of the characteristics of the diode. Hence when a station is accurately tuned in, there is no potential diiference between points A and A, and the outer control grid-cathode bias potential of hexode 92 is determined only by the voltage drop across resistance Slt.

Hexode valves have the following important property: the mutual conductance from, say, the inner grid to the anode may be varied by varying the bias on the outer control grid, and since variations in mutual conductance are accompanied by corresponding changes in amplification factor, the amplication factor with respect to the inner grid may be varied by varying the outer control grid-cathode potential difference, for example.

Now it is known that in a circuit comprising a thermionic valve having a non-reactive load, such as a resistance, connected in its anode circuit, there appears in the grid-cathode circuit of the valve a reflected capacity dependent upon the magnitude of the anode to grid capacity and upon the voltage ampiii'lcation of the Valve. In the arrangement shown in Fig. 3, the hexode 92 has a non-reactive load comprising resistance 9S, and its inner grid-cathode circuit is connected in parallel with the oscillator tuning condenser 16. The reiiected capacity referred to above is effectively in shunt across the tuning condenser lil,V and the tuning of the receiver is thus dependent upon the magnitude of the reiiected capacity.

The latter may be varied by varying the voltage amplication of the hexode, and this in turn may bc made to depend upon the inner grid-cathode potential difference. It will now be shown that this potential difference is dependent upon the extent to which the receiver is offtune with respect to a desired signal, so that the tuning of the receiver is made to adjust itself automatically to compensate for a certain degree of mistuning; that is to say, pull-in tuning is effected.

It will be clear that if the receiver is tuned only approximately to a station, so that, for eX- ample, the actual frequency to which the receiver is tuned is slightly lower than the signal frequency, the resulting intermediate frequency will be, say, less than the value corresponding to a condition of precise tuning, assuming that the local oscillation frequency is always higher than the signal frequency. In these circumstances, the potential difference set up across condenser 8 is larger than that set up across coil 8l, and more current ows in diode 83, 8d than flows in diode 86, 84; point A thus becomes more negative than point A', and the outer control grid bias of hexode 92 is increased in the negative sense.

On account of this increase in negative bias, the amplification factor of the hexode is decreased, and the reflected capacity is also decreased. Since the reflected capacity is in shunt with the tuning condenser 'l of the local oscillator circuit 64, the frequency of the local oscillation is thus increased, the intermediate frequency is correspondingly increased, and the potential difference between points A and A is reduced. The effect continues until the potential difference between points A and A becomes zero, in which circumstances the intermediate frequency has its correct value, and the receiver may be regarded as being in tune, although in fact the signal frequency circuits remain slightly mistuned.

If on the other hand the receiver is mistuned to a frequency slightly above the signal frequency, the potential difference set up across coil tl is greater than that set up across condenser 3b, and the potential of point A lthus becomes more positive than that of point A. The magnication factor of hexode 92, and hence the reflected capacity, is thus increased, and the local oscillation frequency is therefore decreased.

It will be noted that the change in oscillator tuning capacity corresponding to a certain degree of mistuning is a larger fraction of the oscillator capacity when the receiver is tuned to a station at the short wave end of the medium wave broadcast band, for example, than when the receiver is tuned to a station of longer wavelength, since in the former case the oscillator tuning capacity is much less than in the latter case.

'I'he efficiency with which pull-in tuning operates in this arrangement is therefore greater when receiving stations of short wavelength than when receiving long wave stations, and means are accordingly provided for obviating or reducing this variation in efliciency.

It has been pointed out that the anode of hexode 92 is connected to its inner grid by condenser 96, which serves to supplement the natural capacity between these electrodes, so that an adequate reflected capacity may be obtained. The condenser 96 is preferably made variable, and is preferably ganged tothe tuning control of the receiver in such a manner that it has a greater value when the receiver is tuned to a long wave station than when the receiver is tuned to a short wave station. In this way, it can be arranged that the reected capacity for a given degree of mistuning is a substantially constant fraction of the oscillator tuning capacity, so that pull-in tuning of substantially constant eciency is obtained.

Another way of achieving the same result is to arrange that the resistance 93 has a larger value when the receiver is tuned to a long wavelength than when it is tuned to a shcrt wavelength. Resistance Q3 may accordingly be made adjustable, or may be replaced by a plurality of resistances and a suitable switch for connecting one or another of these resistances in circuit. The adjusting or selecting means respectiveiy are preferably ganged to the tuning control.

A combination of the two methods may be ernployed if desired; in this case, either condenser 92 or resistance 93 may be made continuously variable and ganged to the tuning control, the remaining element, that is, the resistance or condenser respectively, being arranged to have a suitable diferent value for each of the wave-ranges to be covered; the change from one value to the other may be effected by means of the waverange changing switch of the receiver.

It will be appreciated that the application of pull--in tuning to a superheterodyne receiver is less complicated than its application to so-called straight receivers, since in the case of the superheterodyne, it is only necessary to control the tuning of one tuned circuit, that is to say, the circuit controlling the frequency of the local oscillation; furthermore, the discriminating circuit, which determines the sense in which com pensaticn for mistuning is applied (and in the case of the receiver described in Fig. 3 is the circuit comprising condenser 80 and coil 3l) may be xedly tuned to the intermediate frequency of the receiver. if desired, however, the receiver described in Fig. 3 may be modied by arranging that compensation for mistuning is also applied to the signal frequency circuits.

From the point of View of its function in assisting pull-in-tuning, the hexode 92 may be regarded as simulating a variable capacity, since in a manner such as that described, to simulate a variable capacity, a valve arranged to simulate a variable impedence of any other suitable kind may be employed. For example, a valve arranged so that its anode-cathode impedance may be varied to control the effective inductance, and hence the tuning of a resonant circuit may be employed.

Furthermore, other nemote control systems than the Wheatstone bridge system described may be employed; one such system which is illustrated in Fig. 4 comprises two electricgmotors, of which one, numbered H3, is arranged at the remote point, while the other H4, is arranged at the receiving'point, the motors being so constructed and connected together that movement of the rotor H5 of motor H3, in response to motion of a tuning knob HB, produces a corresponding movement of the rotor Hl of the motor H4. The rotor H1 is coupled to a tuning member of a receiver H8, and approximate tuning of the receiver is thus obtained from the remote point. The receiver H8 may be of the kind shown in Fig. 2 or 3 for example. In this system, the torque to which the rotor Ill is subjected varies as the square of the phase difference betweenl the two rotors, and hence the torque for small phasel differences is extremely small. Remote control is Very diil'cult to achieve with such a system unless pull-in tuning is employed according to the present invention.

1f desired, means may be provided for disconnecting the remote control device to enable the receiver to be tuned directly, that is, by the direct manual rotation of the tuning condenser spindle. Means may also be provided for disconnecting the loudspeaker from the receiver during the operation of tuning.

Other modifications of the arrangements described, within the scope of the appended claims, will readily occur to those versed in the art.

yI claim:

1. Apparatus for receiving modulated carrier oscillations, comprising an adjustable resonant circuit, a tuning member for adjusting said circuit over a wide frequency range, tuning control means associated indirectly with said tuning member for adjusting said member to diiferent frequencies of said range and tuning said circuit approximately at each adjustment for the reception of a desired carrier, fine-tuning means, means for automatically operating said latter means when there is a discrepancy of less than a predetermined amount between the frequency to which said circuit is tuned by said control meansY and the frequency to which said circuit should be tuned for the reception of said desired carrier, and means for deriving radio-frequency oscillations from said desired carrier and for feeding said oscillations to said automatically operating means for causing said fine tuning means to reduce said discrepancy said deriving means comprising a pair of tubes having a common input circuit and a common output circuit, said automatic operating means being connected to the said common output circuit, said common input circuit being arranged to rectify said oscillations, and means for deriving modulation voltage from the rectified oscillations in said common input circuit.

2. Apparatus for receiving modulated carrier oscillations, comprising an adjustable resonant circuit, a tuning member for adjusting said circuit, biasing means for biasing said tuning member into one extreme setting, re-setting means for urging said tuning member into the other extreme setting, a plurality of separately operable selecting devices for holding said tuning member against the action of said biasing means at any of a plurality of pre-determined settings intermediate said eXtreme settings, thereby tuning said circuit approximately for the reception of a desired carrier, a sourcevof light adjustable in position with said tuning member thereby to produce independent visible indications at said plurality of settings, fine-tuning means adapted to be operated automatically when there is a discrepancy of less than a pre-determined amount between the frequency to which said circuit is tuned when said tuning member is held at one of said intermediate settings and the frequency to which said circuit should be tuned for the reception of said desired carrier, and means adapted tobe fed with radio-frequency oscillations derived from said desired carrier for operating said fine-tuning means when fed by said oscillations, thereby automatically reducing said discrepancy. i

3. Apparatus for receiving modulated carrier oscillations, comprising an adjustable resonant circuit, a tuning member for adjusting said circuit over a wide frequency range, a fine-tuning device associated with said circuit, tuning control means associated indirectly With said member for adjusting said member to different frequencies of said range and tuning said circuit approximately at each adjustment for the reception of a desired carrier, fine-tuning means adapted to be operated automatically when there is a discrepancy of less than a pre-determined amount between the frequency to which said circuit is tuned by said control means and the frequency to which said circuit should be tuned for the reception of said desired carrier, and rectier means adapted to be fed with radiofrequency oscillations derived from said desired carrier for causing said ne-tuning means to operate when fed by said oscillations, thereby automatically adjusting said fine-tuning device to reduce said discrepancy said rectifier means comprising an pair of electron discharge tubes having a, common input circuit constructed and arranged for full wave rectification, means for deriving modulation voltage from the input circuit, a common output circuit for said pair of tubes, and said ne-tuning means being connected to said output circuit.

4. In combination with a radio receiver of the type having a main tuning device constructed to tune the receiver to different carrier frequencies of a relatively wide frequency range, a station selector mechanism for automatically adjusting said tuning device and including a plurality of push button members for predetermining the adjustment of the tuning device to said different frequencies, a tuning device supplemental to said main tuning device for accurately tuning said receiver at the predetermined adjustments of the main tuning device, means, responsive to received carrier energy as the main tuning device approaches each predetermined adjustment, for actuating said supplemental tuning device to perform accurate tuning and a light source mechanically associated with said main tuning device for simultaneous adjustment therewith with respect to said plurality of push button members whereby a plurality of independent, visible indications appear at settings of said tuning device corresponding to said members.

5. In combination with a radio receiver of the type having a main tuning device constructed to tune the receiver to different carrier frequencies of a relatively wide frequency range, a station selector mechanism for automatically adjusting said tuning device and including a plurality of push button members for predetermining the adjustment of the tuning device to said different frequencies, a tuning device supplemental to said main tuning device for accurately tuning said receiver at the predetermined adjustments of the main tuning device, and means, responsive to received carrier `energy as the main tuning device approaches each predetermined adjustment, for actuating said supplemental tuning device to perform accurate tuning, said responsive means comprising a pair of electron discharge devices having a common rectification input circuit and a common output circuit, means connected to said input circuit to derive audio voltage therefrom, means in said output circuit for deriving a unidirectional voltage from said carrier energy, and said supplemental tuning device including a member connected to said common output circuit and responsive to said voltage.

6. In combination with a radio receiver of the type having a main tuning device constructed to tune the receiver to different carrier frequencies of a relatively Wide frequency range, a station selector mechanism for automatically adjusting said tuning device and including a plurality of push button members for predetermining the adjustment of the tuning device to said different frequencies, a tuning device supplemental to said main tuning device for accurately tuning said receiver at the predetermined adjustments of the main tuning device, and means, responsive to received carrier energy as the main tuning device approaches each predetermined adjustment, for actuating said supplemental Ytuning device to perform accurate tuning, said main tuning device comprising a gang of variable condensers, said supplemental tuning device including an adjustable reactance electrically associated with at least one condenser of said gang and an electrical lamp mechanically associated with the rotor element of said gang whereby the position of the lamp is varied with adjustment of the rotor element with respect to said push button members.

HERBERT JACKSON. 

